Inductive resonance has been used to transfer energy in free space. Such systems commonly utilize a resonant circuit to transfer the energy for charging. When energy is stimulated at the frequency of the resonant circuit, the output of the resonant circuit may amplify the energy, provided that the impedance is kept at a low enough level. As the impedance reduces in the resonant circuit, the gain also reduces. The quality (Q) of the resonant circuit is dependent upon its inductance (L), capacitance (C), and resistance (R). Various issues associated with inductive resonance include changing loads caused by changing positions of the antenna in the system and changes in the impedance of the circuit.
Although advances have been made in the field of charging electronic devices, improvements are still needed to enhance existing charging systems.